Process for the preparation of magnesium-thorium alloy



March 20, 1962 J. N. REDING, JR 3,026,196

PROCESS FOR THE PREPARATION OF MAGNESIUM-THORIUM ALLOY Filed Dec. 8. 1959 IN VEN TOR. John N. Rea 1'09, Jr:

Mai/W4 Rfllfldflii Patented Mar. 20, 1962 This invention relates to a method of reducing thorium fluoride and more particularly concerns the reaction of thorium fluoride with molten magnesium metal in amount in excess of that corresponding to the stoichiome tric relationship ThF +2Mg- 2M gF -l-Th whereby thorium is liberated in situ with the excess of the molten unoxidized magnesium metal and a magnesium-thorium ailoy is formed.

This application is a continuation-in-part of copending application Serial No. 795,512, filed February 25, 1959.

Heretofore magnesium-thorium alloys have been prepared either (I) by melting together magnesium metal and thorium metal or (2) by reacting thorium fluoride, admixed with a salt blend such as that described in U.S. Patent 2,678,267, with an excess of molten magnesium metal. However, in the first case the cost of pure thorium metal detracts from the desirability of alloying the metals directly. In the second case previous reduction methods of thorium salts have not been entirely satisfactory because of low reduction and recovery eificiency. In carrying out the reduction of thorium fluoride as described in the above mentioned U.S. patent in which a saline melt containing thorium fluoride admixed with potassium chloride and/ or sodium chloride is contacted with molten magnesium metal, magnesium fluoride which is formed as the reduction proceeds enters the saline melt and causes it to become extremely viscous or putty-like or even harder. As a consequence of the increased viscosity of the saline melt the reduction efficiency of the remaining thorium fluoride therein is thereafter limited as it is quite difficult to bring about intimate contact between the saline melt and the molten metal. Further, to worsen the problem, upon agitating the reaction mixture a great deal of the molten metal becomes entrapped in the saline phase while intimately mixed therewith and forms a heavy sludge. The salt and metal phases are not readily separable from the sludge While the metal is in either the molten or solidified state so that substantial portions of magnesium-thorium alloy are lost in the process and the thorium values can be recovered from the sludge only by extensive processing as by wet chemical treatment. Thus while an entirely satisfactory and useful alloy is obtained using the process of US. Patent 2,678,267, a substantial proportion of the thorium alloy produced in the process is lost.

Another problem encountered in the preparation of magnesium-thorium alloy is the loss of thorium fluoride by conversion to the oxide during blending into a fused saline composition.

It is therefore an object of the invention to provide a method of reducing thorium fluoride with an excess of molten magnesium whereby magnesium-thorium alloy formed by the method is not substantially entrapped in the saline phase.

It is another object of the invention to provide a method of reducing thorium fluoride with an excess of molten magnesium whereby good reduction efliciencies are obtained.

Another object of the invention is to provide a method of reacting a salt blend containing thorium fluoride with an excess of molten magnesium whereby the saline phase retains good fluidity at a temperature in the range of 700 to 1000 C.

Still another object of the invention is to avoid the loss of thorium fluoride during the formation of a saline melt in the process of preparing a magnesium-thorium alloy.

Other objects and advantages will become apparent as the description of the invention proceeds.

The invention is based upon the discovery that by admixing thorium fluoride with lithium fluoride and an alkaline earth metal fluoride selected from the group consisting of calcium fluoride, magnesium fluoride and mixtures thereof, to form a suitably low melting saline mixture, and by reacting the saline mixture with an excess of molten magnesium metal, thorium fluoride is efliciently reduced to the metal. The so-formed thorium metal becomes alloyed with unoxidized molten magnesium metal and the saline phase remains fluid and readily separable from the metal phase as by settling or stratifying while at reaction temperatures.

In carrying out the invention according to one mode thereof, thorium fluoride is first admixed with the requisite amount of a salt blend, consisting of lithium fluoride and alkaline earth metal fluoride, to take up the thorium fluoride, when heated, to form a saline melt. The said requisite amount when mixed with the thorium fluoride contains suflicient lithium fluoride to yield a lithium fluoride to thorium fluoride ratio by weight in the range of 0.15 to 0.75. It is desirable to employ a blend of lithium fluoride with alkaline earth metal fluoride which is fusible at a sufliciently low temperature e.g., 700 to 1000 C., at which molten magnesium is not subject to excessive vaporization. The selection of a suitable salt blend is best understood with reference to the single figure of the annexed drawing in which is shown a phase diagram of the simple condensed ternary system which has been plotted on a triangular coordinate scale in terms of mole percent of each of the three components. A similar diagram has been published by W. E. Roake in Journal of the Electrochemical Society, 104, 662 (1957). in the said phase diagram heavy solid lines known as boundary curves and labeled X, Y and Z, indicate melt compositions at which two solid phases are in equilibrium with a liquid phase upon cooling the melt compositions to appropriate temperatures. The intersection of lines X, Y and Z indicates the ternary eutectic composition. The eflect of the additional variable, temperature, is illustrated on the same two-dimensional diagram by means of a number of selected isothermal lines which are shown in the drawing and identified as to temperature. Isothermal lines are shown as broken lines except for those lines employed in defining ranges of compositions useful in the practice of the invention. Such isothermal lines or portions thereof are covered by a heavy solid line. Along a given isothermal line a composition at the indicated temperature will have a solid and a liquid phase in equilibrium.

Referring now to the said drawing it can be seen that a salt composition within the area ABCDE satisfies the requirement of having a sufficiently low melting point. However, as magnesium fluoride is itself formed in the reduction reaction and enters the saline melt and as the formation during the course of the reduction reaction, of a saline melt having a melting point above the reaction temperature should be avoided, it is to be preferred that the composition employed contain initially, for practical reasons, an amount of magnesium fluoride which is less than about 40 mole percent of the salt blend and at least 10 mole percent of lithium fluoride. Further, because of the expense of lithium fluoride, it is generally desired to use a combination of salts rather than the pure lithium salt initially. Therefore the preferred range of composition of the salt blend is defined by the area of the drawing FGHHK and an even more preferred range of composition permitting the use of less salt blend and reaction temperatures below about 900 C. is defined by the area of the drawing FGLMN.

For the most satisfactory operation the saline blend employed should be of suflicient amount and of such composition as to dissolve all of the magnesium fluoride formed during the reaction at the reaction temperature. Or stated differently, the composition of the saline melt formed during the reaction should remain within the composition limits of the liquid phase for the reaction temperature employed, e.g., composition limits as defined in the drawing by a given set of isothermal lines.

If all of the magnesium fluoride is not dissolved by the saline melt the melt will become increasingly Viscous. and entrap a considerable amount of the molten metal phase. On the other hand the use of considerably more salt blend than necessary to retain fluidity of the saline melt at reaction temperature is also disadvantageous. The use of a large excess of salt blend (1) makes thorough mixing of the salt and metal phases more diflicult (2) results in more thorium fluoride remaining unreacted in the salt phase, and (3) results in more metal phase being entrapped in the saline melt since some entrapment appears inevitable in any case.

The reaction temperature employed obviously has a great deal to do with the composition limits of permissible salt blends. At lower temperatures, that is, below about 700 C., a considerable amount of calcium fluoride and/or lithium fluoride is required to maintain fluidity of the saline melt upon formation of magnesium fluoride as the reaction proceeds. On the other hand at quite elevated temperatures, such as temperatures above 1000 C., thorium reduction efficiency becomes less favorable and magnesium metal has a greater tendency to vaporize. It is to be preferred that a reaction temperature in the range of 750 to 900 C. be employed.

To avoid the loss of reduced thorium metal and magnesium-thorium 'alloy which tend to become dispersed as discrete globules and entrapped in the residual saline melt during agitation of the reaction mixture in the process of the invention, it has been found advantageous to add barium fluoride (BaF to the said saline melt to promote coalescence of the globules of metal containing thorium and thereby permit the globules to escape the saline melt and join the main mass of molten metal. Generally it is convenient to incorporate the BaF in the dry salt blend which is admixed with the thorium fluoride and thereafter fused. An effective proportion of BaF is from about 5 to 8 percent by weight based on the combined weights of the salt blend and the thorium fluoride, although a higher proportion of BaF may be employed, if desired. Use of 6.5 percent by weight of BaF based on the said combined weights, generally assures coalescence of at least 95 percent of the metal globules normally suspended in the molten salt phase.

The thorium-containing salt mixture employed in the invention may be heated in a suitable vessel, for example, an iron or steel melting pot, together with an amount of magnesium in excess of the proportions indicated in the foregoing equation as regards the ratio of magnesium to thorium fluoride, the number of moles of magnesium being generally in the range of from 5 to 1000 times the number of moles of thorium fluoride. If desired, the thorium-containing salt mixture may be first fused and maintained at a temperature sufficient to melt magnesium, e.g., 700 to 1000 C., and magnesium added to the saline melt, or the saline melt may be poured into a melting pot containing molten magnesium, or the thorium fluoride, salt blend and magnesium may be simply heated together to a temperature in the range of 700 to 1000 C. In any case, to avoid conversion of thorium fluoride to thorium oxide it is to be desired that the salts and the melting pot be dried as by heating at temperatures in the range of 225 to 350 C. to remove superficial moisture and that the reduction be carried out either in an atmosphere of an inert gas such as argon or under a suitable protective saline flux cover such as a conventional magnesium foundry flux.

The highest recovery ei'ficiencies of thorium are obtained by adding BaF to the salt blend, and, in addition, employing an anhydrous thorium tetrafluoride. Anhydrous thorium tetrafluoride may be obtained by drying thorium fluoride at an elevated temperature in the range of 550 to 600 C. in an atmosphere of anhydrous hydrogen fluoride or by converting thorium oxide or thorium oxalate to the fluoride by treatment with anhydrou hydrogen fluoride at a similar elevated temperature.

While the thorium-containing salt mixture and magnesium are in contact and while they are both in the molten state they are agitated and mixed together as by mechanical stirring or by rocking the melting pot. After a reaction period of from about 15 minutes to 2 hours depending on the quantity of reaction mixture and the manner of agitating it, the mixture is allowed to stratify for a few minutes while still molten, e.g., for 5' to 30 minutes, and the floating magnesium-thorium alloy is decanted into a suitable mold or simply allowed to solidify in the melting pot. Upon allowing the so-treated mixture to solidify in the melting pot and removing it from the pot it is found that the phase separation is so well defined that the solidified metal alloy can generally be separated from the salt phase by a few sharp blows applied at the interface by means of a hammer and chisel.

While it is possible to regulate the relative densities of the magnesium-thorium alloy and the saline melt formed in the practice of the invention so that the saline melt is of lower density, that is, upon using a high thorium to magnesium ratio while using a light saline melt that contains a high proportion of lithium fluoride, it is preferred that the operation be so carried out that the saline melt is the heavier. One reason for this preference is that the use of a larger proportion of magnesium is conducive to obtaining better reduction efficiencies. Another reason is that thorium oxide is sufliciently heavy that it will not remain suspended in a light saline melt but will settle therefrom during the practice of the invention into any lower phase such as a heavy molten alloy phase and contaminate the lower phase. In any event, the formation of a saline melt and a molten alloy of about equal density is to be avoided in the practice of the invention.

EXAMPLES In each test in a series of tests made according to the practice of the invention a thorium fluoride-containing salt mixture and pieces of magnesium were charged into a steel boat disposed inside a steel shell. The shell was heated by an electrical resistance furnace and was also adapted to operation at reduced pressures as well as under an inert gas atmosphere. The steel boat was provided with a handle in the form of a 1 inch steel pipe which served also as a thermocouple Well. One end of the pipe was welded to the steel boat and the other end protruded outside the shell through a suitable stufling box so that the boat could be rocked back and forth to mix the reactants. After the charged boat was positioned in the shell the system was evacuated and the temperature of the boat brought to and maintained at about 300 C. for three hours to drive off superficial moisture from the materials. The shell was then filled with argon and the temperature of the boat was raised to and maintained at the desired reaction temperature. The so-heated materials were then mixed by rocking the boat back and forth during a reaction period. After the reaction period the product was allowed to stratify for 15 minutes, then the boat was withdrawn into a colder section of the shell and cooled under an argon atmosphere. The metal and the salt melt solidified as separate phases with a sharp boundary between the two. The steel shell was opened calculations it is assumed that the alloy entrapped in the sludge is of the same composition as the recovered alloy although generally the entrapped alloy is found to be higher in thorium than the recovered alloy.

and the solidlfied contents were removed from the boat. 5 The results 1n the table show that both a greatly 1m- In each case the metal was eas1ly separated from the salt proved reduct1on of thor1um tetrafiuonde and recovery of by means of a ch1sel. Thereafter samples of the so-prethor1um alloy are obtalned upon uslng a salt blend con pared magnesium-thor1um alloys were analyzed for ta1n1ng l1th1um fluoride and an alkahne earth metal fluothorium content. Reaction condltrons and compos1t1ons ride according to the practice of the 1nvent1on.

Table Thorium Salt Blend Reaction Percent Percent Percent Test No. Fluoride, Th/Mg Temper- Time, Th in Th Re- ThFi Type ature, Hr. Alloy covered Reduced Composition LiF/Th C.

A 23 800 2 13.7 se 66 A 24 900 2 14.5 61 7 B 23 800 2 16.2 75 86 A 54 900 2 28.6 73 75 o 43 800 2 26.2 s1 84 D .43 800 2 27.5 87 90 A .24 7' 2 15.0 73 75 A 37 800 2 22.0 74 so A 54 800 2 20.7 77 81 A 82 800 2 36.4 72 A 1 50 800 2 47.8 40 67 A 23 800 2 13.8 60 as A 21 800 2 15.0 73 75 D 43 800 2 27.5 87 90 A 23 800 2 13.8 so as A 22 800 2 17.3 75 93 A 22 300 2 3.9 17 19 A 22 800 x 4.3 20 21 A 800 34; 15.0 39 4s A 26 700 2 4.3 10 18 A 24 800 2 3.9 17 1s A 23 800 2 1.7 8 s A=commercial grade ThFmrHgO, z=about 1. B=ThF1-:H2O dried in HF atm. at 600 C O=ThF4 prepared by reacting T1102 n11 hr at 550 0. D=ThFi prepared by reacting Th(C2Ol)2 with 1113 at 550 C.

reacted as Well as test results are listed in the table as tests 1 through 16.

Also shown in the table are the results of additional tests made to serve as a comparison with the present invention. In the comparison tests, listed as tests 17 through 22, reductions were carried out using thorium fluoride-potassium chloride blends and a thorium fluoridemagnesium chloride blend as well as thorium fluoride in the absence of other salts. In a particularly direct comparison of methods, tests 16 and 17 were made simultaneously using a double boat reactor so that both reaction mixtures were exposed to identical conditions.

As used herein the quantities percent thorium recovered and percent thorium fluoride reduced are calculated as follows:

Percent Th reeovered=100 weight alloy recoved percent Th in alloy Weight ThF X percent Th in ThF used Percent Th fluoride reduced= 100 calculated Weight alloyXpercent Th in alloy Weight ThF percent Th in THE; used where calculated weight alloy (wt. A) is obtained from the expression:

Wt. A=Weight of magnesium employed-(percent Th In alloyX wt. AX

+percent Th in al1oy wt. A

In additional reductions carried out to illustrate the practice of the invention there was employed a pot-type reactor which consisted of an 8 inch diameter pipe which was provided with a closure at the bottom and flanged at the top and closed with a cover consisting of a complernentary flanged section equipped with fittings for evacuating the reactor and introducing argon gas, and with an agitator adapted to extend about to the bottom of the reactor when the cover was in place. The reactor was adapted to be heated in a standard gas-fired foundry setting. In one run 10 pounds of magnesium 3.3 pounds of commercial grade ThF -H O containing 71% of thorium, 1.3 pounds of calcium fluoride and 1.4 pounds of lithium fluoride were placed together in the above described reactor and dried under vacuum at a temperature of 300 C. for 3 hours. Then the reactor was filled with argon and heated to and maintained at 800 C. The agitator was turned on and the contents of the reactor were mixed for 1 hour. The reaction mixture was allowed to stratify for 15 minutes after which heating was stopped and the reactor was opened and the contents cast into two separate molds. In pouring the contents from the reactor the magnesium-thorium alloy, being the lighter phase, came over first and was cast into an ingot separately from the saline melt. The so-cast ingot weighed 10.8 pounds and contained 14.8 percent thorium. This corresponds to a thorium recovery of 68 percent and a thorium fluoride reduction efficiency of 71 percent.

In an additional test carried out using the above described reactor 10 pounds of magnesium, 4.3 pounds of commercial grade ThF -H O, 1.05 pounds lithium fluoride and 0.97 pound of calcium fluoride were reacted together in the same manner and under the same conditions as the foregoing test. The resulting magnesium-thorium alloy was similarly cast into an ingot. The ingot weighed 11 pounds and was found to have a thorium content of 19.8 percent. This corresponds to a thorium recovery of 71 percent and a thorium fluoride reduction eificiency of 77 percent.

In an additional test carried out using the above described pot-type reactor, pounds of magnesium, 4 pounds of anhydrous thorium tetrafluoride, 1 pound of lithium fiuoride,-0.95 pound of Cal-* and 0.52 pound of barium fluoride were reacted together in the same manner and under the same conditions as the foregoing test except that the drying period was limited to 2 hours at 100 C., the reaction period was 1% hours and the time for stratification was increased to 30 minutes. The resulting magnesium-thorium alloy was similarly cast into an ingot. The ingot weighed 11.6 pounds and was found to have a thorium content of 21.1 percent corresponding to a thorium recovery of 80.7 percent and a thorium fluoride reduction efliciency of 84.1 percent.

In still two additional tests of the method of the invention carried out using the above described pottype reactor 10 pounds of magnesium, 4 pounds of anhydrous thorium tetrafluoride, 1 pound of lithium fluoride, 0.95 pound of CaF and 0.52 pound of barium fluoride were reacted together in the same manner and under the same conditions as the foregoing test except that the reaction period was reduced to 1 hour. In each test the resulting magnesium-thorium alloy was similarly cast into an ingot. In the one test the ingot weighed 11.4 pounds and was found to contain 22.5 percent thorium corresponding to a thorium recovery of 85 percent and a thorium fluoride reduction efiiciency of 90.8 percent. In the other test the ingot weighed 115 pounds and was found to contain 22.4

percent of thorium corresponding to a thorium recovery of 85.6 percent and a thorium fluoride reduction efliciency of 90 percent.

In a further test of the method of the invention carried out using the pot-type reactor described hereinabove 10 pounds of magnesium, 4.05 pounds of anhydrous thorium tetrafluoride, 1 pound of lithium fluoride, and 0.96 pound of CaF were reacted together in the same manner and under the same conditions as the foregoing test. The resulting magnesium-thorium alloy was similarly cast into an ingot. The ingot weighed 10.9 pounds and was found to have a thorium content of 22.6 percent corresponding to a thorium recovery of 79 percent and a thorium fluoride reduction efliciency of 90 percent.

Among the advantages of the method of the invention is the avoidance of the use of hygroscopic salts.

What is claimed is:

1. A method of preparing magnesium-thorium alloy which comprises subjecting thorium tetrafluoride in admixture with a saline blend to the reducing action of molten magnesium at an elevated temperature in the range of 700 to 1000 C. whereby metallic thorium liberated in the reduction becomes alloyed with magnesium, said saline blend consisting of lithium fluoride and an alkaline earth metal fluoride selected from the group consisting of magnesium fluoride, calcium fluoride and mixtures thereof, said saline blend being within the composition range represented by the area FGHIJK of the drawing and the number of moles of magnesium being from 5 to 1000 times the number of moles of thorium tetrafluoride.

2. The method as in claim 1 in which the ratio by weight of lithium fluoride in the saline blend to the thorium tetrafluoride admixed therewith is in the range of 0.15 to 0.75.

3. The method as in claim 1 in which the thorium tetrafluoride and the saline blend are subjected to the reducing action of molten magnesium at a temperature in the range of 750 to 900 C. and the said saline blend is within the composition range represented by the area FGLMN of the drawing.

4. The method as in claim 1 in which the thorium tetrafluoride admixed with the saline blend is substantially anhydrous thorium tetrafluoride.

5 A method of preparing magnesium-thorium alloy which comprises subjecting horium tetrafluoride in admlxture with a saline blend to the reducing action of molten magnesium at an elevated temperature in the range of 700 to 1000 C. whereby metallic thorium liberated in the reduction becomes alloyed with magnesium, said saline blend consisting of barium fluoride, and a combination of lithium fluoride and an alkaline earth metal fluoride selected from the group consisting of magnesium fluoride, calcium fluoride and mixtures thereof, the composition of said combination of lithium fluoride and an alkaline earth metal being within the composition range represented by the area FGHIJK of the drawing, the proportion of said barium fluoride being from about 5 to 8 percent by weight based on the weight of the said admixture, and the number of moles of magnesium being from 5 to 1000 times the number of moles or thorium tetrafluoride.

6. A method of preparing magnesium-thorium alloy which comprises subjecting thorium tetrafluoride in admixture with a saline blend to the reducing action of molten magnesium at a temperature in the range of 700 to 1000 C. for a time suflicient for a substantial portion of the thorium tetrafluoride to be reduced to the metal in situ whereby metallic thorium so-liberated becomes alloyed with magnesium, said saline blend consisting of' lithium fluoride and an alkaline earth metal fluoride selected from the group consisting of magnesium fluoride, calcium fluoride and mixtures thereof, said saline blend being within the composition range represented by the area FGHIJK of the drawing and the number of moles of magnesium being from 5 to 1000 times the number of moles of thorium tetrafluoride.

7. A method of preparing magnesium-thorium alloy which comprises heating thorium tetrafluoride together with a saline blend consisting of lithium fluoride and an alkaline earth metal fluoride selected from the group consisting of magnesium fluoride, calcium fluoride and mixtures thereof, said saline blend being selected from the composition range represented by the area FGHIJK of the drawing, at a sufficiently elevated temperature whereby a saline melt is formed and subjecting the soformed saline melt to the reducing action of molten magnesium at a temperature in the range of 700 to 1000 C. whereby metallic thorium so liberated becomes alloyed with unoxidized molten magnesium, the number of moles of magnesium being from 5 to 1000 times the number of moles of thorium tetrafluoride.

8. A method of preparing magnesium-thorium alloy which comprises heating thorium tetrafluoride together with a saline blend consisting of barium fluoride, and a combination of lithium fluoride and an alkaline earth metal fluoride selected from the group consisting of magnesium fluoride, calcium fluoride and mixtures thereof, at a sufliciently elevated temperature whereby a saline melt is formed and subjecting the so-formed saline melt to the reducing action of molten magnesium at a temperature in the range of 700 to 1000 C. whereby metallic thorium so liberated becomes alloyed with unoxidized molten magnesium, the composition of said combination of lithium fluoride and an alkaline earth metal being within the composition range represented by the area FGH'IJK of the drawing, the proportion of said barium fluoride being about 6.5 percent by weight based on the combined weight of the said saline blend and the thorium tetrafluoride, and the number of moles of magnesium being from 5 to 1500 times the number of moles of thorium tetrafluoride.

References Cited in the file of this patent UNITED STATES PATENTS Saunders May 11, 1954 Keller Aug. 12, 1958 OTHER REFERENCES 

1. A METHOD OF PREPARING MAGNESIUM-THORIUM ALLOY WHICH COMPRISES SUBJECTING THORIUM TETRAFLUORIDE IN ADMIXTURE WITH A SALINE BLEND TO THE REDUCING ACTION OF MOLTEN MAGNESIUM AT AN ELEVATED TEMPERATURE IN THE RANGE OF 700 TO 1000*C, WHEREBY METALLIC THORIUM LIBERATED IN THE REDUCTION BECOMES ALLOYED WITH MAGNESIUM, SAID SALINE BLEND CONSISTING OF LITHIUM FLUORIDE AND AN ALKALINE EARTH METAL FLUORIDE, SELECTED FROM THE GROUP CONSISTING OF MAGNESIUM FLUORIDE, CALCIUM FLUORIDE AND MIXTURES THEREOF, SAID SALINE BLEND BEING WITHIN THE COMPOSITION RANGE REPRESENTED BY THE AREA FGHIJK OF THE DRAWING AND THE NUMBER OF MOLES OF MAGNESIUM BEING FROM 5 TO 1000 TIMES THE NUMBER OF MOLES OF THORIUM TETRAFLUORIDE. 